Overstock.com always seems to have a selection of PIII Thinkpads, the T series being small and light (and pretty compatible with other OSes). They start around the low 400’s for an 800MHz and go up from there.

Some places I’ve used have been the venerable compgeeks.com and PCSurplusOnline.com, the latter of which often has N%-off deals if you get on their spam list. Then there’re the Wal-Mart.com laptops, and TheInquirer is doing a semi-weekly roundup of the best deals on new stuff. (Seems like you can get something from the AXP-M range for around $500 these days, but I haven’t been looking closely.)